1. Field of the Invention
The field of the present invention is that of condensation or polycondensation processes in the field of silicones.
2. Description of Related Art
It is well known that it is possible to polycondense silanols, silanediols or polysiloxanediols in the presence of various compounds which accelerate the polycondensation by acting as dehydrating agent or as catalyst (cf., for example, the treatise by Walter Noll: Chemistry and Technology of Silicones; 1968 edition, pp. 211 to 218).
Mention may be made, among dehydrating agents, on the one hand, of acid compounds, such as sulfuric acid, phosphoric acid or acid chlorides, and, on the other hand, of nonacid compounds, such as isocyanates, boric esters, and the like. All these compounds have to be used in a stoichiometric proportion.
Mention may be made, among catalysts, of halogenated acids, basic catalysts, such as alkaline hydroxides, and amines, such as triethylamine. Finally, it is also possible to activate the polycondensation reaction by using organometallic compounds of metals such as lead, tin, zirconium, aluminum, calcium, sodium or potassium.
In addition, these various catalysts are supposed to also promote the reaction for equilibration of the siloxane bonds by opening the latter with subsequent polymerization of the bonds thus released and, for this reason, formation of volatile cyclic compounds (Journal of Polymer Science, 59, 259-269, 1962).
Condensation processes in the field of silicones can also involve, on the one hand, α,ω-dihydroxylated polydiorganosiloxanes and, on the other hand, silicon-comprising compounds of the di-, tri- or tetraalkoxysilane type. In this type of condensation reaction, the prior technical literature abounds in examples of catalysts also referred to as functionalization catalysts. The following may be mentioned, without implying limitation: amines, inorganic oxides, organic titanium derivatives, titanium/amine combinations, hydroxylamines, aluminum chelates, carbamates and oximes.
All these known catalysts suffer from unacceptable disadvantages. In particular, amines result in low reaction kinetics, even with highly reactive alkoxysilanes, such as alkoxysilanes of formula ViSi(OCH3)3 with Vi=vinyl group. Moreover, amines have an unpleasant smell and are toxic. They contaminate the reaction medium and destabilize the finished products.
Catalysts based on titanium and on hydrocarbon groups, such as tetraisopropoxytitanium, have the harmful effect of causing gelling of the medium, which is a particular nuisance at an industrial stage.
Functionalization catalysts of the potassium acetate type (U.S. Pat. No. 3,504,051) or sodium acetate type (U.S. Pat. No. 3,563,241) are also known; as are carboxylic acid/amine mixtures, as taught by patent FR 2 604 713. Such catalysts suffer from being relatively corrosive and thus difficult to handle. In addition, they do not make it possible to significantly improve the crosslinking kinetics.
U.S. Pat. No. 5,026,811 describes the crosslinking of silicone resins of the polymethylphenylsiloxane type by employing an organometallic catalyst composed of a mixture of alkali metal carboxylates, of an alkali metal carbonate or bicarbonate and optionally of an ammonium carboxylate. The alkali metal selected can, for example, be lithium and the carboxylates selected can, for example, be acetates or 2-ethylhexanoates. These catalytic mixtures exhibit the disadvantage of not being directly soluble in a silicone medium and of giving rise to residues which are difficult to remove.
More recently, functionalization catalysts (SiOH/SiOR condensation) have been provided which are formed of hydroxides of alkali metals, such as sodium or potassium (patents EP 457 693 and U.S. Pat. No. 5,196,497) or else of lithium (U.S. Pat. No. 5,079,324). This novel class of catalysts is targeted at replacing the carboxylates, which appear to a person skilled in the art to be unsuitable, inefficient and defective, when they are employed alone in this application. Unfortunately, these catalysts of the alkali type have the defect of their aggressive nature, which is expressed especially at high temperatures, for example of the order of 100° C. This is because the high alkalinity which they generate brings about decomposition of the reactants and products of the condensation reaction. In addition, this basicity complicates the handling of the reaction medium. Moreover, these inorganic hydroxides, which are insoluble in the silicones, require the use of polar solvents, which bring about the appearance of the regenerated forms of the alkali metal concerned. These forms would cause a phenomenon of lysis of the polymer and/or crosslinked product obtained (reversion).
Thus it is that, in order to improve the catalysis of this type of condensation reaction, European patent application No. 0 564 253 teaches the use of a catalyst composed of a lithium-based organometallic compound which does not require the use of polar or aprotic solvent. More specifically, the catalysts disclosed are lithium silanolates or alkyllithiums, such as tert- or n-butyllithium. It turns out that these known catalysts are still likely to bring about, under hot conditions, decomposition of the reactants and products. This teaching comes within the continuation of the general tendency to improve the catalysis of the SiOH/SiOR reactions by abandoning the carboxylate route. It is necessary to observe that the improvements obtained remain unsatisfactory, with regard to the stability obtained and indeed even with regard to the kinetics obtained.
Metals are thus good catalysts of the condensation reaction, even at ambient temperature. Their effectiveness depends very clearly on the temperature, but these conditions may then promote decomposition side reactions, such as demethylation. Tin, for example in the dibutyltin dilaurate form, is commonly used for its ability to catalyze these reactions at low temperatures, in particular in the case of the network preparation. Unfortunately, this metal is highly toxic and will eventually have to be replaced.
One of the major lines of research in the field of polysiloxanes thus relates to the search for catalysts simultaneously combining performance, specificity and nontoxicity.
Furthermore, platinum/carbene complexes are known as catalysts for the hydrosilylation of a polyorganosiloxane comprising ≡Si-vinyl units by means of a polyorganosiloxane comprising ≡Si—H units. Mention may be made, by way of example, of application PCT WO-A-02/098971, which describes a silicone composition which can be crosslinked to give an elastomer by hydrosilylation in the presence of metal catalysts based on carbenes. This composition comprises:                a polyorganovinylsiloxane poly(dimethyl)-(methylvinyl)siloxane,        a polyorganohydrosiloxane,        a platinum catalyst formed by a complex (C3) or (C4) of following formulae:        
                optionally a crosslinking inhibitor, and        optionally a filler.        
Hydrosilylation is neither a condensation reaction nor a polycondensation reaction. In such a hydrosilylation, the carbene acts only as ligand of the platinum and thus does not act as catalyst. Furthermore, the carbenes used as ligands of catalytic metals are also made use of in other fields than those of the silicones. Thus, patent EP-B-0 971 941 describes catalysts based on ruthenium and osmium/carbene complexes for the thermal metathesis of cycloolefins.
In addition, a paper by J. L. Hedrick et al. which appeared in 2002, (JACS, 124, No. 6, pp. 914-915, 2002), teaches that N-heterocyclic carbenes can be used as catalyst for the polymerization of cyclic esters. More specifically, 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene was tested as catalyst for the polymerization of L-lactide, ε-caprolactone and β-butyrolactone in the presence of an alcohol used as initiator. From the mechanistic viewpoint, the authors believe that, since the pKa is very high (pKa=24, measured in DiMethyl SulfOxide), the carbene is highly nucleophilic and can thus attack the cyclic ester monomer to give an activated entity capable of attacking the alcohol of the initiator or of the growing chain according to an initiation/propagation process represented below:

A more recent and fuller paper (cf. JACS, 125, No. 10, pp. 3046-3056, 2003) describes the preparation of carbene in situ to avoid problems of hydrolysis. The promoters used are of thiazolium, imidazolium and imidazolinium type, respectively giving thiazolecarbene, imidazol-2-ylidenecarbene and imidazolin-2-ylidenecarbene catalysts, when they are brought into contact with potassium tert-butoxide. Polymerization tests show that the first family of catalysts does not make it possible to obtain high molecular weights in a profitable and selective manner.
The carbenes obtained by deprotonation of an imidazolium salt in the presence of a strong base have recently been described for the synthesis of polyorganosiloxane silicones by polymerization by the opening of the ring(s) and/or redistribution of linear or cyclic polyorganosiloxanes (patent application FR 2 864 543).